1. Field of the Invention
The present invention relates to a chip-formed solid electrolytic capacitor having a structure wherein a projecting anode lead is not provided for an anode member and a method of producing the chip-formed solid electrolytic capacitor.
2. Description of the Prior Art
In recent years, miniaturization and increase in number of functions of electronic apparatus have progressed remarkably, and as a result of the progress, electronic parts which allow high density mounting are now demanded. Also solid electrolytic capacitors are demanded to allow high density mounting, and principally, chip-formed capacitors of the resin molded type for surface mounting are used as such. Referring to FIG. 1, a chip-formed capacitor of the resin molded type includes a capacitor element 13 on which an anode lead 14 is implanted uprightly. A cathode lead terminal 16 is mounted on the capacitor element 13 by a conductive bonding agent 15, and an anode lead terminal 17 is bonded to the anode lead 14. The end sides of the cathode lead terminal 16 and the anode lead terminal 17 are exposed to the outside along the outer face of a resin molding member 18 for outer packaging. In the chip-formed capacitor of the resin molded type, however, since the lead terminals 16 and 17 for the cathode and the anode must be mounted and led out to the outer face of the resin molding element 18, the proportion of the capacitor element 13 to the entire volume of the chip-formed capacitor cannot be made very high. Consequently, the chip-formed capacitor of the resin molded type is unsatisfactory for high density mounting and is complicated in the process of fabrication. Thus, chip-formed solid electrolytic capacitors of the construction in which no resin molding member is used are developed energetically as disclosed, for example, in Japanese Patent Publication No. Sho-61-31609 (JP, B2, 61-31609) by K. Mitsui.
FIG. 2 is a side elevational sectional view showing the construction of a chip-formed Solid electrolytic capacitor which does not employ a resin molding member. A chip-formed capacitor of the type mentioned is formed in the following manner. In particular, an anodic oxidation film or coat 22, a solid electrolyte layer 23 and a cathode layer 24 are successively formed on the surface of an anode member 21 on which an anode lead 29 is implanted uprightly. Then, a resin layer 28 for outer packaging is formed on the entire outer peripheral face of the anode member 21 except a face which opposes the face at which the anode lead 29 is led out. Thereafter, a metal catalyzer is applied to the face of the anode member 21 at which the anode lead 29 is led out and the opposed face to form a pair of plated layers 25a and 25b made of nickel or some other suitable material. Further, solder layers are deposited on the plated layers 25a and 25b to form a cathode outer electrode layer 26 and an anode outer electrode layer 27, respectively. Finally, the anode lead 29 is cut. A valve metal such as aluminum or tantalum is used for the anode member 21.
With the conventional chip-formed capacitor, in order to increase the electrostatic capacitance value per volume, the projection amount of the anode lead remaining after cutting must be reduced while maintaining the reliability in connection between the anode lead and the anode outer electrode layer. Japanese Utility Model Laid-Open Application No. Hei-2-137025 (JP, U, 2-137025) by H. Oka discloses a technique wherein the projection amount of an anode lead can be made very small by connecting a cut face of the anode lead and an anode external electrode layer to each other by way of a plated layer. With the method of Oka, since electric connection is established at the cut face of the anode lead, special attention must be paid to maintenance of the reliability in connection. Japanese Patent Laid-Open Application No. Hei-3-97212 (JP, A, 3-97212) by Y. Saiki discloses another technique wherein a plated catalyzer metal layer, a plated layer and a solder layer are successively formed to form an external anode layer and the external anode layer and an anode lead are connected to each other by a metal-to-metal junction to achieve improvement in close contactness and good matching in coefficient of thermal expansion between the anode lead and the external anode layer to assure improved reliability in connection. Further, Japanese Patent Laid-Open Application No. Hei-4-99011 (JP, A, 4-99011) by Y. Saiki discloses a further technique wherein an alloy layer is interposed between an anode lead and an anode outer electrode layer to achieve improvement in reliability in connection.
In the conventional chip-formed solid electrolytic capacitors described above, while an anode lead projects from an anode member and a cut face of the anode lead or a face of the anode lead from which an oxide coating has been removed is connected to an anode outer electrode layer, in order to assure a high degree of reliability in connection, the anode lead must project by a length greater than a fixed length. This is a disadvantage of the conventional chip-formed solid electrolytic capacitors from the point of view of miniaturization and/or improvement in volume efficiency.